It's not quite right to portray it entirely as "ideological purity bullshit".
Mozilla's trademark policy forbade using their trademarks with custom builds that weren't Mozilla binaries, or at least with ones that might contain non-Mozilla modifications. (I can't remember the exact details of what was allowed.) In addition to that kind of creating a bit of a conundrum with the idea of free or open source software, it would also have forbidden Debian from custom-patching Firefox with their own security fixes.
The Debian policy was to keep stable releases static and only make security and bug fixes to packages in the stable release. Firefox didn't have extended support releases at the time, so the only ways of keeping the browser patched were to constantly update to new releases or to backport patches to older releases, so Debian was still patching Firefox by themselves. The Mozilla trademark policy made it impossible for them to keep doing that, or to even maintain their freedom to do that, and keep using the Firefox trademarks.
Debian is just generally very strict about licensing and playing by the rules. (They're also strict about adhering to their free software guidelines, which might be considered ideological. And yes, the distribution is opinionated, but that doesn't mean the rationale is always ideological.)
Some other distributions, especially smaller and less formal ones, might have just ignored the entire trademark policy and kept doing whatever they wanted despite it being potentially in violation of the conditions of Mozilla trademark use. Or they might have just shrugged and decided they didn't care about being able to patch the browser themselves anyway, especially if they were a rolling distro.